If you know the switch is good it's easy enough to check whether the plunger on the switch is closing the contacts. Just put the shifter in reverse and check continuity across the switch.
Some info:
By Richard Rowe, Contributor
While the muscle car-era Muncie M20/22 may have made headlines, the fact is that far more GM vehicles received the unsung SM420/465 box than its more famous cousin. The SM420 and SM465 were ungainly-looking things, shaped more like a four-piece toaster than a four-speed transmission. However, these transmissions were tremendously strong, offered a very low "granny gear" first gear for rock-crawling and stump-pulling and outlived the M20/22 by decades.
Transmissions
GM produced the "top-loader" SM420 transmission from 1947 to 1967, and replaced it with the upgraded SM465 in 1968. The SM465 had higher gearing than the SM420: 6.55 first gear, 3.58 second, 1.57 third and a 1.00 fourth. It also used a stronger case, stronger input-output shafts and came in four basic versions. All 1968 to 1979 4WD boxes used a 10-spline input shaft and output shaft; later 4WD transmissions used a 10-spline input and 32-spline output shaft. All two-wheel-drive versions used a 10-spline input and 35-spline output, but some came with a larger 1 1/2-inch input shaft (compared to the other versions' 1 1/8-inch shaft).
Light Sensors
The SM465 used an integrated back-up light switch that received its input directly from the transmission internals instead of from the shifter or shift rails -- which the top-loader doesn't have -- like most other transmissions. The sensor plugs into the side of the transmission, near the back where you'd expect the reverse gear to be.
Identification
Some later versions of this transmission also used an integrated neutral safety switch, which cuts power to the starter whenever the transmission is in gear. This safety switch is very similar to the reverse-light switch, but located closer to the front of the transmission. If you see two sets of sensor wires protruding from the transmission, the reverse-light switch is the one closest to the driveshaft or transfer case.
Failure
Reverse light switches can fail in a number of ways, but failure generally begins as an intermittent event and quickly goes total. The reverse light circuit is fairly simple; power goes from the fuse box, through the switch and directly to the back-up lamps. Check the back-up bulb first, then the fuse and finally the connections before attempting to replace the reverse light switch. The reverse light switch can be very difficult to remove on trucks without a removable transmission tunnel, since the switch needs about three inches of clearance to slide out of the transmission. You might consider cutting a small access port in your transmission tunnel, then riveting a piece of sheet-metal over the hole to ease installation and future servicing.
